In physics, resonance is the tendency of a system to oscillate at larger amplitude at some frequencies than at others. These are known as the system's resonant frequencies (or resonance frequencies). At these frequencies, even small periodic driving forces can produce large amplitude vibrations, because the system stores vibrational energy. When damping is small, the resonant frequency is approximately equal to the natural frequency of the system, which is the frequency of free vibrations. Resonance phenomena occur with all types of vibrations or waves
http://en.wikipedia.org/wiki/Resonance